JSON Schemas: Add missing internal not_registered property

[priethor]

What

Adds a not_registerd property to the internal entity subschema.

Why

When listing a SCF entity through the abilities that is not actually registered (e.g., due to a duplicated slug), the entity is returned with the not_registered property, and the schema validation fails.

Steps to reproduce

1. Duplicate a post type so that it has the same slug and a different SCF key, so that the copy will not be registered
2. Use the Ability to list post types; it will fail on trunk but work on this branch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 1.17%. Comparing base (41a727c) to head (97f29e1).
⚠️ Report is 2 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##             trunk    #234   +/-   ##
=======================================
  Coverage     1.17%   1.17%           
  Complexity    4416    4416           
=======================================
  Files          261     261           
  Lines        27017   27017           
=======================================
  Hits           318     318           
  Misses       26699   26699           

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[github-actions]

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Props priethor.

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